I am disturbed by the oft-repeated claim that Abraham, Issac, and Jacob followed all the details of Torah before it was given to us at Sinai. I suspect that it is a case of later generations projecting their own ideals back onto their ancestors. I realize that today this is generally an unpopular opinion among Orthodox Jews, but I would like to know: are there any respected rabbis over the years who have doubted this concept?
Rama in teshuvos yud. Says only Avraham kept the Torah not rest of Avos. The Rambam malachim implies that the Avos didn't keep the whole Torah, only a select few mitzvos. Which is difficult since the gemara says Avraham kept the whole Torah. Minchas Asher Vayishlach offers few suggestions.. one is that das zikeinim birieshis 18.8 brings midrash that implies Avraham didn't keep the Torah...
There is a gur aryeh in Vayigash 46.10 who suggests that only Avraham kept the whole Torah. But the rest only kept the posotive commands not the negative. Also Tosafos in Baba basra 141a asks why didn't Yitschak marry his sister bakol if a goy can marry his sister. The simple understanding of Tosafos is that Yitschak didn't keep the Torah.
It depends who you consider a respected rabbi, but I hope that Rambam meets the criteria. In his letter to Hasdai Ha-levi1 he writes:
There can be no doubt that the patriarchs, as well as Noah and Adam, although they did not observe the Torah imperatives, were not consigned to Purgatory (gehenom) but ascended to the highest degree of virtue for having attained what is necessary for perfection.
(p. 108 in Stitskin translation Letters of Maimonides)
1. It is perhaps noteworthy that R. Yitzchak Sheilat classifies this letter in the category of uncertain Maimonidean authorship.
Rashi, based on Midrash wrote that Abraham observed the entire Torah. This seems reasonable since G-d blessed Isaac “because Abraham listened to my voice and kept my charge, commandments, statutes, and laws.” (See Genesis 26:4 and 5).
Nachmanides agreed with Rashi (see his commentary on Leviticus 18:25). He felt that Abraham knew the Torah by prophecy. Accordingly, the patriarchs, although not obligated, asked G-d if they could keep the entire Torah, even though it would not be revealed centuries later.
Maimonides disagreed. According to the Rambam, Abraham did not have the Torah. He felt that Abraham had only the Seven Noahide Laws plus eight for circumcision (eight mitzvot). It is possible that the rabbinical opinion that the patriarchs observed the Torah was hyperbolic; designed to highlight the importance of Torah observance. It is not meant to be taken literally. If Abraham kept the Torah the Torah would have mentioned it.
 See Genesis Rabbah 79:7, 92:4, and 95:2, the Babylonian Talmud, Yevamot 21.
 See Mishneh Torah (Hilkhot Shmitah v’Yovel, 13:11).